The invention relates to an electronic ballast including a DC-AC converter for igniting and supplying a gas discharge lamp, and in particular to such a converter having two input terminals to be connected to a DC voltage source. The input terminals are interconnected by means of a series arrangement which includes a load circuit comprising at least the discharge lamp and an induction coil, and a first semiconductor switching element including a freewheel diode. The load circuit is bridged by a circuit including a second semiconductor switching element and also having a freewheel diode, the semiconductor switching elements being provided with control circuits for rendering said switching elements alternately conductive. A converter of this type is known from U.S. Pat. No. 4,647,820.
The above-mentioned semiconductor switching elements are rendered conducting and non-conducting by means of their respective control electrodes. The freewheel function can be provided using a type of semiconductor element having an integral diode element or using a separate diode element arranged in parallel therewith.
The aforesaid Patent describes a half bridge converter with a transformer which is present in the load circuit (in which the lamp is incorporated). This transformer has two secondary windings which form part of the control circuits of the semiconductor switching elements. The switching elements are rendered alternately conducting and non-conducting by means of the transformer and the control circuits, respectively. It has been found that it is difficult to adjust the oscillation frequency of the system in a reproducible manner to a fixed value due to the use of a transformer. This is a drawback, notably when the system is used in lamps which are manufactured in a bulk manufacturing process. Moreover, the transformer described is voluminous and costly, and the associated control circuits comprise a comparatively large number of components. The integration of the circuit into a compact discharge lamp (such as an "SL" lamp) is then difficult.